1. Field of the Invention
The present invention generally relates to cordless telephone sets, and more particularly to a cordless telephone set having a function of detecting a call while a built-in battery built is being charged.
2. Description of Related Art
FIG. 1A shows a related cordless telephone set 10. The cordless telephone set 10 is configured as follows. A radio interface unit 1 communicates, via a radio channel, with a host device (FIG. 1B) or another cordless telephone set connected to a telephone line TL coupled to an exchange EX. A portable telephone system is made up of the portable telephone set 10 and the host device. A speech receiver 2 receives a speech signal transmitted by another subscriber and received via the host device, the radio channel and the radio interface unit 1. An amplifier 3 amplifies the speech signal received by the speech receiver 2. An amplified speech signal output by the amplifier 3 drives a speaker 4, which converts the amplified speech signal into an acoustic speech signal. A microphone 5 converts an acoustic speech signal into a corresponding electric speech signal. An amplifier 6 amplifies the electric speech signal from the microphone 6, and outputs an amplified speech signal to a speech transmitter 7. The speech transmitter 7 generates a radio wave from the amplified speech signal. The radio wave is transmitted via the radio interface unit 1, a radio channel and the host device. A signal receiver 8 receives call control signals output by the above exchange and received via the telephone line, the radio channel, the host device and the radio interface unit 1 and transmits these control signals to a processor 11. Examples of these call control signals are a calling signal, a disconnect signal, a response signal, and a clear signal. A signal transmitter 9 receives call control signals from the processor 11 and transmits these call control signals to the exchange via the radio interface unit, the radio channel, the host device and the telephone line. Examples of the call control signals are a call signal, a select signal of a caller, a disconnect signal, a response signal, and a clear signal. The processor 11 controls the entire operation of the cordless telephone set 10. More specifically, the processor 11 rings a sounder 14 in response to receipt of the calling signal from the signal receiver 8, and outputs the select signal based on a dialed number input by means of a ten-key keypad 12 to the signal transmitter 9. Further, the processor 11 detects the ON/OFF states of a hook switch 13, and outputs the calling signal, the response signal, the disconnect signal, the clear signal or the like to the signal transmitter 9.
Electricity necessary to operate the above-mentioned structural elements is generated by a battery 15, and is supplied thereto via a switch 19 and a power supply line 20 or 21. The power supply line 20 carries electricity necessary to ring a sounder 14 and thereby notify the subscriber of receipt of a call to the radio interface unit 1, the signal receiver 8 and the processor 11. The power supply line 21 carries electricity necessary to operate all of the other structural elements.
The switch 19 selects one mode from among three modes, that is, a pause mode P, a standby mode S, and an active mode A. In the pause mode P, an output terminal of the battery 15 is completely disconnected from all the structural elements of the cordless telephone set 10. In the standby mode S, the battery 15 is connected to only the power supply line 20 via the switch 19. In the active mode A, the battery 15 is connected to the power supply line 21, and the power supply line 20 via a diode 22.
When the active mode A is selected by the switch 19, electricity generated by the battery 15 is supplied to all of the structural elements, and hence the cordless telephone set 10 is maintained in an active state in which it can receive a call and transmit a call. It will be noted that a large amount of energy is consumed in the cordless telephone set 10 in the active mode A. When the standby mode S is selected by the switch, electricity is supplied to only the radio interface unit 1, the signal receiver 8 and the processor 11. Hence, in the standby mode S, an amount of energy is consumed smaller than that consumed in the active mode A. When the pause mode P is selected by the switch 19, electricity is not supplied to the structural elements at all. Hence, energy stored in the battery 15 is consumed only slightly. In the pause mode P, it is impossible to notify the subscriber of receipt of a call.
With the above in mind, it is desired that the switch 19 be maintained in the pause mode P when there is no need to use the cordless telephone set 10 and that the switch 19 be maintained in the standby mode P when the subscriber is to be notified of receipt of a call. When the subscriber responds to a call, he or she switches the mode of the switch 19 from the standby mode S to the active mode A. This switching operation is carried out if the subscriber calls another subscriber.
Generally, a battery charger is provided for the cordless telephone set. For example, a battery charger is built in the host device. FIG. 2 shows a battery charger 30 for use in the cordless telephone set 10 shown in FIG. 1A. The battery charger 30 has a power unit 31, a commercial power supply plug 32, and a pair of terminals 33. The power unit 31 generates a D.C. current from commercial power supplied via the plug 32. A pair of the terminals 33 is respectively connected to a pair of terminals 16 of the cordless telephone set 10 shown in FIG. 1 when the battery 15 is charged. The D.C. current generated by the power unit 31 is supplied to a charging circuit 17 via the terminals 33 and 16. The charging circuit 17 generates a battery charging current from the received D.C. current and outputs it to the battery 15 via a diode 18. Energy consumed, for example, in the standby mode S is compensated for by means of charging by means of the charging circuit 17.
When the battery 15 is charged in the standby mode S, the subscriber can be notified of receipt of a call. However, there is a possibility that the charging of the battery 15 is carried out in the pause mode P and nevertheless the subscriber wishes to be notified of receipt of a call. In this case, in conventional technology the subscriber is not notified of receipt of a call.